Kent's captain and lower-order basher put on an unbeaten 108 for the sixth wicket after Hampshire had reduced them to 70-5 in their chase of 172-6, in which James Vince top scored for England's two-time T20 champions.
Northeast anchored the innings with 60 not out, but it was Blake's career-best 71 off 30 balls, including a six to win it with four balls to spare, which rescued a victory which takes the Spitfires to the top of the South Division.
Captain Luke Wright (49) and overseas Mahela Jayawardene (36) got the Sharks off to a good start before Chris Jordan, released from the England squad to play for his county, thumped 37 off 23 balls to take Sussex to 175-6.
Jordan got some tap with the ball, going for 46 in his four overs, but crucially ran out Jim Allenby, who had led Somerset's response with a well-paced 79, to leave the home side's lower order with too much to do at the death.
In a clash between two of the tournament favourites, Yorkshire made the most of an Alex Hales-less Notts to restrict their hosts to 143-7, with Samit Patel (43) and Steven Mullaney (38) doing their bit in the absence of the England batsman.
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Mullaney also took 2-27 with the ball, but Vikings skipper Andrew Gale batted through for 68 not out to see his side home in the 19th over with six wickets in hand.
Durham head Yorkshire on net run rate after two games of the campaign having beaten Lancashire by 16 runs at at Old Trafford.
After Mark Stoneman (57) and former England captain Paul Collingwood (30) put Durham into a strong position at 105-2, the Lightning pegged them back to 155 all out inside the 20 overs, but Ashwell Prince's 78 was a solo effort in response.
Elsewhere in the North Division, defending champions Birmingham Bears got off the mark with victory in their derby against Worcestershire Rapids, while there was also a first win for Leicestershire Foxes against local rivals Derbyshire Falcons.
Ireland wicket-keeper Niall O'Brien was the star for Leicestershire as he took three catches, including two in two balls off Jigar Naik, in Derby's effort of 163-6, before hitting 47 not out after brother Kevin O'Brien had earlier made 31.
After a long delay, Ravi Bopara picked up where he left off to see Essex comfortably home with a knock of 81 which included five fours and five sixes.
Leicestershire Foxes (164-3) beat Derbyshire Falcons (163-6) by seven wickets
Sussex Sharks (175-6) beat Somerset (170-8) by five runs
Essex Eagles (145-5) beat Glamorgan (144-8) by five wickets
Durham Jets (155 a/o) beat Lancashire Lightning (139-9) by 16 runs
Yorkshire Vikings (146-4) beat Nottinghamshire Outlaws (143-7) by six wickets
Birmingham Bears (144-5) beat Worcestershire Rapids (126 a/o) by 18 runs
Kent Spitfires (178-5) beat Hampshire (172-6) by five wickets